This invention relates to wideband code division multiple access (WCDMA) for a communication system and more particularly to signal-to-interference ratio estimation of WCDMA signals.
Present code division multiple access (CDMA) systems are characterized by simultaneous transmission of different data signals over a common channel by assigning each signal a unique code. This unique code is matched with a code of a selected receiver to determine the proper recipient of a data signal. These different data signals arrive at the receiver via multiple paths due to ground clutter and unpredictable signal reflection. Additive effects of these multiple data signals at the receiver may result in significant fading or variation in received signal strength. In general, this fading due to multiple data paths may be diminished by spreading the transmitted energy over a wide bandwidth. This wide bandwidth results in greatly reduced fading compared to narrow band transmission modes such as frequency division multiple access (FDMA) or time division multiple access (TDMA).
New standards are continually emerging for next generation wideband code division multiple access (WCDMA) communication systems as described in Provisional U.S. Patent Application No. 60/082,671, filed Apr. 22, 1998, and incorporated herein by reference. These WCDMA systems are coherent communications systems with pilot symbol assisted channel estimation schemes. These pilot symbols are transmitted as quadrature phase shift keyed (QPSK) known data in predetermined time frames to any receivers within range. The frames may propagate in a discontinuous transmission (DTX) mode. For voice traffic, transmission of user data occurs when the user speaks, but no data symbol transmission occurs when the user is silent. Similarly for packet data, the user data may be transmitted only when packets are ready to be sent. The frames are subdivided into sixteen equal time slots of 0.625 milliseconds each. Each time slot is further subdivided into equal symbol times. At a data rate of 32 thousand symbols per second (ksps), for example, each time slot includes twenty symbol times. Each frame includes pilot symbols as well as other control symbols such as transmit power control (TPC) symbols and rate information (RI) symbols. These control symbols include multiple bits otherwise known as chips to distinguish them from data bits. The chip transmission time (Tc), therefore, is equal to the symbol time rate (T) divided by the number of chips in the symbol (N).
Referring to FIG. 1, there is a simplified diagram of a mobile communication system. The mobile communication system includes an antenna 100 for transmitting and receiving external signals. The diplexer 102 controls the transmit and receive function of the antenna. Multiple fingers of rake combiner circuit 104 combine received signals from multiple paths. Symbols from the rake combiner circuit 104 are applied to a bit error rate (BER) circuit 110 and to a Viterbi decoder 106. Decoded symbols from the Viterbi decoder are applied to a frame error rate (FER) circuit 108. Averaging circuit 112 produces one of a FER and BER. This selected error rate is compared to a corresponding target error rate from reference circuit 114 by comparator circuit 116. Detector circuit 118 produces an output signal corresponding to the comparison. This output signal and a feedback signal from delay circuit 120 are added by circuit 122 to produce a signal-to-interference ratio (SIR) reference signal on lead 124.
Pilot symbols from the rake combiner 104 are applied to the SIR measurement circuit 132. The SIR measurement circuit produces a received signal strength indicator (RSSI) estimate from an average of received pilot symbols. The SIR measurement circuit also produces an interference signal strength indicator (ISSI) estimate from an average of interference signals from base stations and other mobile systems over many time slots. The SIR measurement circuit produces an SIR estimate from a ratio of the RSSI signal to We ISSI signal. This SIR estimate is compared with a target SIR by circuit 126. Detector circuit 128 produces an output signal corresponding to the comparison that is applied to TPC command circuit 130. The TPC command circuit 130 sets a TPC symbol that is transmitted to a remote base station. This TPC symbol instructs the base station to either increase or decrease transmit power by preferably 1 dB for subsequent transmission.
The diagram of FIG. 2 illustrates the closed-loop transmit power control sequence between of the base station and the mobile system. The base station receives a group of pilot symbols 200 in a time slot 204 from the mobile system. The base station determines an SIR ratio from the pilot symbols 200 and TPC symbol 202 and adjusts transmit power accordingly. This adjusted transmit power is applied to time slot 210 of downlink 220. The time slot 210 is offset from time slot 204 by one-halftime slot or 0.3125 milliseconds, so the mobile system has time to adjust transmit power in response to TPC symbol 208 for the next time slot 218 of uplink 230. The mobile system determines an RSSI estimate from pilot symbols 206 of time slot 210. For high data-rate channels such as 256-1024 thousand symbols per second (ksps), there are preferably eight pilot symbols in each time slot. For low data-rate channels such as 32-128 ksps (FIG. 3) there are preferably four pilot symbols in each time slot. The ISSI estimate includes an average of interference signals over many time slots. The ISSI estimate, therefore, is relatively stable and changes slowly with time. By way of comparison, an RSSI estimate for time slot 310 may include of an average of pilot symbols 308 alone. This small sample produces large variations in the RSSI estimate. For example, for six fingers of rake combiner circuit 104, the RSSI estimate Ŝm for the mtm time slot is given by equation [1]. Here, rk,m,g corresponds to the kth pilot symbol of the mth time slot and the gth finger with the pilot symbol data removed.                                           S            ^                    m                =                              1            16                    ⁢                                    ∑                              g                =                1                            6                        ⁢                          (                                                "LeftBracketingBar"                                                            ∑                                              k                        =                        1                                            4                                        ⁢                                          xe2x80x83                                        ⁢                                          r                                              k                        ,                        m                        ,                        g                                                                              "RightBracketingBar"                                2                            )                                                          [        1        ]            
This RSSI estimate may fluctuate abruptly due to the limited number of pilot symbols available for averaging. The SIR estimate is given by equation [2], where {circumflex over (1)}m is the ISSI estimate for the mth time slot, which is obtained by averaging the interference from many previous time slots. Since the SIR estimate is a ratio of the RSSI to the ISSI estimate, most of the variation of the SIR estimate is due to the RSSI variation. The variation in the SIR estimate produces erratic TPC control and correspondingly large variations in transmit power.                               S          ⁢                      I            ^                    ⁢                      R            m                          =                                            S              ^                        m                                              I              ^                        m                                              [        2        ]            
These large variations in transmit power degrade communications between the base station and the mobile system.
These problems are resolved by a circuit comprising an estimate circuit coupled to receive a plurality of predetermined signals from an external source. Each of the predetermined signals is spaced apart in time. The estimate circuit produces a first estimate signal in response to at least one of the plurality of predetermined signals. An averaging circuit is coupled to receive a data signal and the at least one of the plurality of predetermined signals. The averaging circuit produces an average signal from the data signal and the at least one of the plurality of predetermined signals.
The present invention improves signal-to-interference estimation by averaging pilot symbols and corrected data symbols. Closed-loop power control is improved. A standard deviation of transmit power is greatly reduced, and the link margin of the system is improved.